Line an 8 x 8 baking pan with parchment paper and very lightly grease.
In a small saucepan, add the heavy cream, butter, and salt. Heat it just until it reaches a boil, then remove from heat and set aside. You will need to occasionally stir to prevent skin forming on top of the cream.
In a medium/small saucepan, add the sugar, maple syrup, and water together. Stir until combined, then heat on medium until it reaches 280 degrees F. Do not stir the mixture anymore once you've initially combined it.
Use a wet pastry brush to clean the sides of the pan as needed.
Pour in the cream mixture (do not stir) and boil until it reaches 250 degrees F. Immediately remove from the heat and pour into the baking pan.
Gently top with the chopped hazelnuts.
Leave to cool and firm - at least 4 hours, preferably overnight.